sip.bisv.ru has no NS records.
NS records for this zone are defined at bisv.ru.
NS records for this zone are defined at bisv.ru.
Question and response
Question
dig sip.bisv.ru. NS
Answer
The response has no answer records.
Authority
bisv.ru. 10800 SOA ns1.bisv.ru. support.bisv.ru. 2026030601 86400 900 1857600 10800
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0